How difficult would it be to put in a Green-to-Gold packet now that I'm in my 15-month window?

AECP an green to gold are very different programs. Your reenlistment window has nothing to do with your eligibility to apply to a commissioning program. If you're accepted to a commissioning program (except an AD one) you'll be immediately discharged to join ROTC.
As for Reclass, if you're an initial term Soldier you can still request to reclass in your reenlistment, it's just not guaranteed. Persistence pays off. If you're not an initial term Soldier, 68V is a medical MOS that accepts E4P and E5 and is always under strength. It just takes asking for an exception to policy. The exception is justified if it supports the army's needs and end strength goals.